sorry for the double post but edit button does not seem to work
as for pinion size,
more teeth = higher top end (for tracks with long straights)
less teeth = better acceleration
you should aim to be at max speed for the final part of the straight, if you are not at top speed by then you are sacrificing acceleration for speed that you are not using.
using too small of a pinion will cause you motor/esc to get hot (i think) and this is not a good thing.
disclaimer - i am new to brushless myself if at any point in trying to help you i have made a mistake i am sure someone will pipe up and let you know
hope this helps mike